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Edexcel Geography A Level
What are the two classifications of tectonic hazards?
seismic and volcanic
Describe the global distribution of earthquakes
The main earthquake zones are found along plate boundaries
Roughly 70% of all earthquakes are found in the Ring of Fire in the Pacific Ocean.
The most powerful are associated with convergent or conservative boundaries but rare intra-plate earthquakes can occur.
What is the oceanic fracture zone (OFZ)
a belt of activity through the oceans along the mid-ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea Rift and California.
An oceanic fracture zone is a zone of lithospheric weakness generally perpendicular to the axis of the mid oceanic ridge
What is the continental fracture zone (CFZ)
a belt of activity following the mountain ranges from Spain, via the Alps, to the Middle East, the Himalayas to the East Indies and the Pacific.
Where do a small minority of earthquakes occur?
Along old fault lines e.g. the Church Stretton Fault in Shropshire
Define Seismic hazards
generated when rocks within 700km of the Earth’s surface come under such stress that they break and become displaced
Define a volcanic hazard
associated with an eruption event
When does an intra-plate earthquake occur? and why?
In the middle or interior of tectonic plates and are much rarer than boundary earthquakes. The causes of this are not fully understood but it is assumed that plates have pre-existing weaknesses which become reactivated, forming seismic waves. For example, an intraplate earthquake may occur if solid crust, which has weakened overtime, cracks under pressure.
What is a volcano?
A landform that develops around a weakness in the Earth’s crust from which molten magma, volcanic rock, and gases are ejected or extruded.
Describe the global distribution of volcanoes
There are about 500 active volcanoes and about 50 of them erupt annually. Volcanic hotspots, such as the Ring of Fire, are also situated amongst the centre of pates. This is a localised area of the lithosphere (Earth’s crust and upper mantle) which has an unusually high temperature due to the upwelling of hot molten material from the core. (First theorised by Tuzo Wilson in 1963) ➔ At hotspots, such as the Hawaii hotspot, magma rises as plume (hot rock).
Volcanoes are primarily distributed along tectonic plate boundaries, with the majority found in the "Ring of Fire," a horseshoe-shaped zone surrounding the Pacific Ocean. This region accounts for about 75% of the world’s active and dormant volcanoes due to subduction zones. Other significant areas include mid-ocean ridges (e.g., Iceland), rift valleys (e.g., East African Rift), and hotspots (e.g., Hawaii and Yellowstone).
What is a hazard?
a percieved natural event which poses a potential threat to human life and property
What is a volcanic hotspot?
A localised area of the lithosphere (Earth’s crust and upper mantle) which has an unusually high temperature due to the upwelling of hot molten material from the core. Here magma rises as plume (hot rock)
A hotspot is when one of Earth's outer tectonic plates moves over an unusually hot part of the Earth's mantle (magma/mantle plume) and large amounts of magma rise up, piercing through the plates and producing large volcanic eruptions at the Earth’s surface e.g. Kilauea, Hawaii, Yellowstone
Outline how hotspot volcanism has led to the development of the Hawaiian islands. You should refer to examples volcanic activity present in Hawaii and the types of landforms such as shield volcanoes that are produced. How do landscapes such as the Hawaiian islands change over time.
The Hawaiian Islands were formed by such a hot spot occurring in the middle of the Pacific Plate. While the hot spot itself is fixed, the plate is moving. So, as the plate moved over the hot spot, the string of islands that make up the Hawaiian Island chain were formed.
The Hawaiian Islands form an archipelago that extends over a vast area of the North Pacific Ocean. The archipelago is made up of 132 islands, atolls, reefs, shallow banks, shoals, and seamounts stretching over 1,500 miles from the island of Hawaii in the southeast to Kure Atoll in the northwest.
Some volcanic eruptions are 'intra-plate' meaning there are distant from a plate boundary at locations called mid-plate hotspots, such as Hawaii and the Galapagos Islands.
At these locations:
Isolated plumes of convecting heat, called mantle plumes, rise towards the surface, generating basaltic volcanoes that tend to erupt continuously.
A mantle plume is stationary, but the tectonic plate above moves slowly over it.
Over millennia, this produces a chain of volcanic islands, with extinct ones most distant from the plume's location.
stats on hotspots
About 95% of the world’s volcanoes are located near the boundaries of tectonic plates.
The other 5% are thought to be associated with mantle plumes and hot spots.
What is a mantle plume?
Where do mantle plumes occur?
areas where heat and/or rocks in the mantle are rising towards the surface. A hot spot is the surface expression of the mantle plume.
Mantle plumes occur where there are rigorous rising convection currents. In some locations these can break through overlying plates to create tectonic activity. Some current hotspots are shown on the map below. Hawaii is the most significant 'intra-plate' example.
Hotspot volcanism
How has it led to the formation of the Hawaiian Islands?
does not occur at the boundaries of Earth’s tectonic plates, where other volcanism occurs.
The mantle plumes that form hotspots are thought to be relatively stationary, while tectonic plates are not.
Once the tectonic plate has moved, the once active volcano is now no longer on top of the mantle plume and so becomes extinct.
What are the characteristics of the different sections of the Earth’s structure?
Crust: outside layer of earth made of solid rock (basalt and granite)
There are two types of crust
Oceanic: denser and thinner, mainly basalt
Continental: less dense, thicker, mainly granite
Mantle: below the crust, up to 2900km thick. Consists of hot, dense, iron and magnesium-rich solid rock.
The upper part of the mantle and the crust make up the lithosphere and these are broken up into plates- either oceanic or continental.
Crystals in the transition zone hold as much water as all the oceans on Earth’s surface.- but as hydroxide- how cool!
The Asthenosphere is partially melted- the mechanically weak and ductile region of the upper mantle of earth- below the lithosphere
In between the core and mantle is the Gutenberg discontinuity 2900km- the velocity of seismic waves changes abruptly here
The core: centre of earth
liquid outer- nickel, iron, molten rock
solid inner
Radioactive reactions occur inside the core which produces convection currents in the mantle. This causes the tectonic plates to move.
What was Pangea? and what evidence is there for the fact that the continents we know today as Africa and South America were once joined?
A single supercontinent which comprised of North and South America, Africa and Europe. It was during the late Paleozoic Era until the very late Triassic. 300-200 million years ago. #bringbackPangea2025 #rip
Evidence:
Continental/Jigsaw Fit- there is a similarity in the coastlines of eastern south America and west Africa- this best fits at a depth of 1,000 metres below current sea level. Any gaps or overlaps that could disprove this can be explained by coastal erosion, deposition and eustatic and isostatic changes.
Geological Fit- Both coastlines have ancient rock outcrops (cratons) over 2,000 million years old of the same rock type. A belt of ancient rocks along the Brazilian coast, for example, matches one in West Africa.
Glacial Fit- Indications of widespread glaciation from 380 to 250 million years ago are evident in Antarctica, southern South America, southern Africa, India, and Australia. If these continents were once united around the south polar region, this glaciation would become explicable as a unified sequence of events in time and space. More evidence comes from glacial striations – scratches on the bedrock made by blocks of rock embedded in the ice as the glacier moves. These show the direction of the glacier, and suggest the ice flowed from a single central point.
Tectonic Fit- Fragments of an old fold mountain belt between 450 and 400 million years ago are found on widely separated continents today. Pieces of the Caledonian fold mountain belt are found in Greenland, Canada, Ireland, England, Scotland and Scandinavia. When these land masses are re-assembled the mountain belt forms a continuous linear feature.
Fossil evidence- The plant Glossopteris is a fern that has been found in Africa, Antarctica, Australia and South America. It is used as evidence that these continents must have at some point around 250 million years ago been joined. Mesosaurus is an extinct reptile that has been found in both Africa and South America. As Mesosaurus was a coastal animal, and therefore could not have+ crossed the Atlantic Ocean
Paleomagnetism- see flashcard
Marine deposits- Moreover, the earliest marine deposits along the Atlantic coastlines of either South America or Africa are Jurassic in age (approximately 199.6 million to 145.5 million years old), which suggests that the ocean did not exist before that time.
Along the Wadati- Benioff foci, the depth of waves shows subduction of the denser basaltic oceanic plates into the upper mantle.
What is Continental drift? Who are some people that came up with and some geophysical evidence used to develop these theories?
The surface of the Earth is split up into a series of tectonic plates. These move across the Earth's surface due to convection currents in the mantle. Where the plates meet or rub against each other mountains and volcanoes may form and earthquakes may happen, sometimes causing huge waves called tsunamis.
Alfred Wegener's Continental Drift hypothesis in 1912 that postulated that now-separate continents had once been joined.
Harry Hess- Sea Floor Spreading- identified mid ocean ridges, new sea floor being created
The ideas of Arthur Holmes in the 1930s that Earth's internal radioactive heat was that driving force of mantle convection that could move tectonic plates.
The discovery in 1960 of the asthenosphere, a weak, deformable layer beneath the rigid lithosphere, on which the latter moves.
The discovery in the 1960s of magnetic strips in the oceanic crust of the sea bed; these are palaeomagnetic signals from past reversals of the Earth's magnetic field and prove that new ocean crust is created by the process of sea-floor spreading at mid-ocean ridges. (seafloor spreading and palaeomagnetism occur at constructive margins, where new crust is being made) Vine and Matthews
The recognition of transform (conservative- slide) faults and Hotspots by Tuzo Wilson in 1965.
Describe and explain paleomagnetism
The alternating polarisation of new land created. As magma cools, the magnetic elements within will align with the Earth’s magnetic field, which can alternate over thousands of years. We are due another flip. The climate crisis has an effect here.
Describe and explain sea floor spreading
a geologic process in which tectonic plates—large slabs of Earth's lithosphere—split apart from each other.
a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
Describe and explain slab pull
As oceanic lithosphere cools, it becomes denser and thicker. At a convergent plate boundary the oceanic lithosphere sinks beneath the adjacent plate forming an ocean trench and subduction zone. As a result of its own weight, the descending plate is pulled by gravity through the mantle asthenosphere, which is hotter and less rigid. This force is known as slab pull. It is believed to be the major force driving plate motions.
occurs at destructive margins.
Describe and explain Mantle Convection
Mantle convection describes the movement of the mantle as it transfers heat from the white-hot core to the brittle lithosphere. The mantle is heated from below, cooled from above, and its overall temperature decreases over long periods of time. All these elements contribute to mantle convection.
Convection currents transfer hot, buoyant magma to the lithosphere at plate boundaries and hot spots. Convection currents also transfer denser, cooler material from the crust to Earth’s interior through the process of subduction.
Earth's heat budget, which measures the flow of thermal energy from the core to the atmosphere, is dominated by mantle convection. Earth’s heat budget drives most geologic processes on Earth, although its energy output is dwarfed by solar radiation at the surface.
It was long thought that this resulted in convection currents in the mantle which were responsible for the movement of tectonic plates across the Earth’s surface – indeed this is still the most common idea illustrated in many textbooks and on the internet. However, this theory is now largely out of favour, with modern imaging techniques unable to identify mantle convection cells that are sufficiently large to drive plate movement. Some plate models show that two thirds of the Earth’s surface move faster than the underlying mantle so there appears to be little or no evidence that convection currents in the mantle move plates (apart maybe from some very small plates in unusual circumstances).
Describe and explain elastic rebound theory
Explains how energy is stored in rocks
overtime stresses in the earth build up
creates a locked fault (a fault that is not slipping because frictional resistance is greater than the sheer stress)
stress becomes too much (like 4 a levels) the earth breaks
Rocks bend until the strength of the rock is exceeded
Rupture occurs and the rocks quickly rebound to an undeformed shape
Energy is released in waves that radiate outward from the fault.
generally happen along fault planes or lines of weakness
What are the names of the plates?
seven major and eight minor
Each plate is in motion relative to its neighbours, resulting in geological activity at the plate boundaries. It is also possible, though less common, for geological activity to take place in the middle of plates.
describe and explain rib push/gravitational sliding
Where slab pull is not the main plate driver, ‘ridge push’ is another possibility. As the lithosphere formed at divergent plate margins is hot, and less dense than the surrounding area it rises to form oceanic ridges. The newly-formed plates slide sideways off these high areas, pushing the plate in front of them resulting in a ridge-push mechanism.
How many types of plate boundaries/margins are there? What are they? what occurs at them? Describe and explain
Divergent (constructive)
At divergent plate margins, plates are moving apart and so magma rises through the asthenosphere to the surface of the Earth.
typically occurs along a mid-oceanic ridge, like the mid-Atlantic rift that extends from the north to the south of the Atlantic Ocean.
Long chains of mountains form along these ridges. Due to the varying amount and rate of magma released mid-oceanic ridges vary in shape.
Eruptions along constructive plate margins mainly occur underwater. Pillow lavas are formed as lava is rapidly cooled on the sea floor. In the North Atlantic the extrusion of magma has been so great it created the largest volcanic island in the world, Iceland.
As magma rises the rocks above often form a dome. The lithosphere is put under great stress and eventually fractures along faults. This forms the underwater rift valleys found along mid-oceanic ridges.
Rift zones also occur on land and help explain how continents break up. The continental crust must be thin for rifting to happen. One of the best examples is Iceland’s rift valley, þingvellir. This is where the North American Plate and the Eurasian Plate are separating. A graben or sunken valley has been formed where the crust has been stretched, causing faulting.
East African rift
Corinth rift- youngest rift
Convergent (destructive)
At convergent plate margins, plates are moving towards one another. They can meet in 3 different ways:
oceanic-continental (oceanic subducts and this leads to the formation of an ocean trench- the point where the oceanic plate enters the asthenosphere. Continental crust buckles forming an oceanic trench. Sedimentary rock formed on top of the oceanic crust folds upwards along the edge of continental plate. The continental crust also lifts and buckles and magma is injected from the asthenosphere. This process forms fold mountains of which the Andes and the Rockies are examples. As the oceanic crust subducts the continental crust it melts. The magma rises as it is less dense than the material around it. Large intrusions of magma create uplift, further contributing to the formation of fold mountains. Volcanoes are formed where magma reaches the surface of the Earth.
oceanic-oceanic Where two oceanic plates converge the denser crust subducts the other. This creates a trench. As the oceanic plate descends it melts, and the magma rises forming a volcanic island chain, known as an island arc. The north-west Pacific Ring of Fire has a series of island arcs including the Aleutian Islands.
continental-continental - Where two continental plates meet there is typically no subduction. Fold mountains, such as the Alps and the Himalayas form.
Conservative
Conservative margins are also known as transform faults.
At conservative margins, plates slide past each other, so that the relative movement is horizontal, and classified as either sinistral (to the left) or dextral (to the right). Lithosphere is neither created nor subducted, and whilst conservative plate margins do not result in volcanic activity, they are the sites of extensive shallow focus earthquakes, occasionally of considerable magnitude.
It is possible to see the boundary between plates along a conservative margin. An example of this is the San Andreas fault in California. This is where the North American and Pacific plates slide past each other.
Transform faults are mainly found on the ocean floor, where they offset mid ocean ridges and enable to ocean to spread at different rates. It was through the work of John Tuzo Wilson that these faults were recognised as the connection between the ocean ridges (divergent margins) and ocean trenches (convergent margins).
What is rifting?
What is a megathrust earthquake and where does it occur?
occur at subduction zones at destructive plate boundaries, the earth’s most powerful with Moment magnitudes exceeding 9.0!!
What are the 3 types of fault?
normal- the block above the fault moves down relative to the block below the fault
reverse- the block above the fault moves up relative to the
block below the fault
strike-slip- the movement of blocks along a fault is horizontal
assess the role of convection currents in the theory of plate tectonics
The asthenosphere behaves like a fluid over very long time scales. There are a number of competing theories that attempt to explain what drives the movement of tectonic plates. Three of the forces that have been proposed as the main drivers of tectonic plate movement are:
mantle convection currents: warm mantle currents drive and carry plates of lithosphere along a like a conveyor belt
ridge push (buoyant upwelling mantle at mid-ocean ridges): newly formed plates at oceanic ridges are warm, so they have a higher elevation at the oceanic ridge than the colder, more dense plate material further away; gravity causes the higher plate at the ridge to push away the lithosphere that lies further from the ridge
slab pull: older, colder plates sink at subduction zones because, as they cool, they become more dense than the underlying mantle and the cooler, sinking plate pulls the rest of the warmer plate along behind it
Research has shown that the major driving force for most plate movement is slab pull, because the plates with more of their edges being subducted are the faster-moving ones. However, ridge push is also presented in recent research to be a force that drives the movement of plates.
How is an earthquake formed?
An earthquake are caused by sudden movements near the Earth’s surface (lithosphere) along a fault (zones of pre-existing weakness in the Earth’s crust)
Movements are preceded by a build-up of tectonic strain, which stores elastic energy in crustal rocks (the lithosphere). This generates a locked fault.
Pressure builds up along the fault which can cause deformation of the lithosphere.
When the pressure exceeds the shear strength of the fault, the rock fractures causing a rupture and release of energy where rocks jolt past each other.
This produces a sudden release of energy (seismic waves) that radiate away from the point of fracture (hypocentre).
The brittle crust then rebounds either side of the fracture which is the ground shaking; the earthquake felt on the surface. The lithosphere reverts to the original undeformed shape in a new locked fault.
What are seismic waves? and the four types
Seismic waves can travel both along the surface and through the layers of the earth. There are three types of waves –
P waves (primary) body: cause the immediate shock. Fastest wave. Can move through solid and liquid. Pushes and pulls the rock it moves through
S waves (secondary) body: longer wavelength and arrives seconds later. Can only move through solid rock (this includes the mantle). Moves rock up and down or side to side.
L waves (love): surface wave. Only travel through the crust causing horizontal movement
R waves: also a surface wave- travel just blow or along the ground’s surface, slower than body waves, rolling movement, especially damaging to buildings
Describe liquefaction
The molecules vibrate in both solids and liquids, but in solids they vibrate in place whereas in liquids they have that much more energy so they can slip and slide over each other too. In an earthquake, the molecules in the solid ground are given enough energy that they are no longer required to vibrate just in place, but are also capable of this ‘liquefaction’ – of sliding around and over each other, just like a liquid. Once the seismic waves are spent and there is no more energy being provided, the molecules go back to behaving like a solid
this affects loose rock and sediment. The seismic waves trigger the ground to lose its load bearing capacity, causing large buildings to settle into the ground, tilt and possibly collapse
Why is the Moment Magnitude scale seen as more accurate than the Richter scale
Unfortunately, many scales, such as the Richter scale, do not provide accurate estimates for large magnitude earthquakes. Today the moment magnitude scale, abbreviated MW, is preferred because it works over a wider range of earthquake sizes and is applicable globally. The moment magnitude scale is based on the total moment release of the earthquake. Moment is a product of the distance a fault moved and the force required to move it. It is derived from modeling recordings of the earthquake at multiple stations. Moment magnitude estimates are about the same as Richter magnitudes for small to large earthquakes. But only the moment magnitude scale is capable of measuring M8 (read "magnitude 8") and greater events accurately.
Magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and about 32 times as much energy would be released). To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT.
Magnitude scales can be used to describe earthquakes so small that they are expressed in negative numbers. The scale also has no upper limit. The largest recorded earthquake occurred along the subduction zone in Chile in 1960. It was a magnitude 9.5 but larger earthquakes may be possible. Fortunately, large earthquakes are much less common than small ones.
What is the Modified Mercalli Intensity scale and why is it criticised?
Another way to measure the strength of an earthquake is to use the observations of the people who experienced the earthquake, and the amount of damage that occurred, to estimate its intensity. The Mercalli scale was designed to do just that The original scale was invented by Giuseppe Mercalli in 1902 and was modified by Harry Wood and Frank Neumann in 1931 to become what is now known as the Modified Mercalli Intensity Scale.
Although the Mercalli scale does not use scientific equipment to measure seismic waves, it has been very useful for understanding the damage caused by large earthquakes. It has also been used extensively to investigate earthquakes that occurred before there were seismometers.
Some factors that affect the amount of damage that occurs are:
the size (magnitude) of the earthquake
the distance from the epicenter,
the depth of the earthquake,
the building (or other structure) design,
and the type of surface material (rock or dirt) the buildings rest on.
What is the difference between an earthquake prediction and an earthquake forecast?
Probabilities and forecasts are comparable to climate probabilities and weather forecasts, while predictions are more like statements of when, where, and how large, which is not yet possible for earthquakes.
What is an earthquake?
An earthquake is the sudden ground motion or vibration produced by a rapid release of stored-up energy
What is the difference between focus (hypocentre) and epicentre of an earthquake:
The location below the earth's surface where the earthquake starts is called the hypocentre and the location directly above it on the surface of the earth is called the epicentre.
What are benioff zones?
The Benioff Zone of earthquakes is caused by the subduction of one tectonic plate under another. The earthquakes at the surface boundary between the two plates are shallow. The subducted plate is forced down deeper causing intermediate earthquakes and as it is forced into the mantle it continues to produce earthquakes (deep earthquakes) until the plate finally is reabsorbed into the mantle at around 700 km or so.
Characteristics of a shallow focus earthquake
Shallow quakes generally tend to be more damaging than deeper quakes. Seismic waves from deep quakes have to travel farther to the surface, losing energy along the way. Shaking is more intense from quakes that hit close to the surface like setting off "a bomb directly under a city,"
Characteristics of a deep focus earthquake
While deep quakes may be less damaging, they're usually more widely felt. Most of the destruction in the Myanmar quake was centered in the tourist town of Bagan where nearly 100 brick pagodas dating back centuries were damaged. At least four people were killed in the Myanmar temblor, which also shattered ancient Buddhist pagodas.
What is a seismometer
A seismometer measures the amount of ground shaking during an earthquake, recording vertical and horizontal movements of the ground on to a seismograph.
Landslide
these occur where slopes are weakened by seismic waves and slide under the influence of gravity
Landslides occur when the shear stress is greater than the shear strength.
70% of all deaths from EQs globally (excluding those from shaking, building collapse, tsunami) are attributable to landslides.
e.g. 2008 Sichuan EQ landslides accounted for 1/3 of all deaths
Shear strength – the force holding the slope together (e.g. lots of trees, shallow slope)
Shear stress – the forces pulling the slope down (e.g. increasing slope angle if building a road)
What is an event profile?
Event profiles can be drawn for any event and help illustrate the great
variation in the nature of tectonic hazards. They are a common way
to compare and contrast different hazards. The typical earthquake
and volcanic profiles tend to differ most in terms of spatial predictability
and frequency.
What are some changes which decrease stability of slopes?
A decrease in shear strength (upslope forces):
Weathering of rock allows water porosity of rocks to increase (freeze-thaw cycles) allowing more water to enter.
Weathering also breaks down rocks into new clay which expands when water is present (dry clay is very firm and stable)
Increasing the water content – raises the water table (can increase stress and at the same time decrease strength)
Animals burrowing – allows soil moisture to drain away
Removing vegetation – vegetation binds the soil thus stabilising slopes, the loss of root networks reduces the cohesion of the soil. Slope failures often occur several years after logging, when root systems decay away.
An increase in shear stress (an increase in the forces attempting to pull a mass downslope):
Construction
An increase in slope angle especially at the base of the slope e.g. undercutting for building a road removes the support for the slope
Increasing weight of slope due to increased water content
Shocks and vibrations from earthquakes or machinery – the shaking causes rearrangement of particles, decreasing porosity à water content change from unsaturated to saturated without adding water.
Tsunamis (another secondary hazard) some stats and how do they appear
77% of all deaths in the 2004 Indian Ocean tsunami were women
Tsunami account for 7% of tectonic disasters but 36% of deaths
Three of the top 10 most deadly disasters in recent years were tsunami (2004 Boxing Day - Banda Aceh, Indonesia; 2008 – Sulawesi, Indonesia; 2011 – Tohoku, Japan) – all at the Pacific Ring of Fire.
‘Tsunami’ is a very large wave which floods areas of the coast.
When they are out at sea, they have a very long wavelength, often more than 100km. They are very short in amplitude, at around 1m in height. They travel very quickly often at speeds of up to 700kph, for example taking less than a day to cross the Pacific.
When they reach land, they rapidly increase in height up to over 25m in some cases. They are often preceded by a localised drop in sea level (drawback) as water is drawn back and up by the tsunami. This is often the first warning of its arrival. It slows down as it approaches a land mass but as the frequency of the wave remains the same, so the height of the wave increases greatly
Describe and explain the two causes of tsunamis
Earthquakes
The most common cause of major tsunami is submarine earthquakes occurring beneath the seabed. The earthquake can cause a vertical displacement of the seabed, displacing water upward, which generates a tsunami at the ocean surface. Horizontal displacements of the seabed (strike-slip faults) do not tend to generate tsunami.
Volcanic collapse
These most commonly involve the eruption, or emergence, of a volcanic island. There are two main mechanisms:
1. Flank collapse: the landslide of one side of volcano into the sea, displacing water. This is often accompanied by a lateral blast.
2. Caldera collapse: where the upper part of a volcano collapses, accompanied by a massive steam eruption as water contacts magma.
The 2018 Sunda Strait tsunami (Indonesia) involved a volcano. The island of Anak Krakatoa is formed of a volcano that emerged in the sea from Krakatoa’s crater (which famously erupted in 1883) in 1927. There was a flank collapse during the 2018 eruption created a submarine landslide and tsunami 13m high. There were 435 deaths, 14,000 injured and 3000 homes
How tsunamis occur via earthquakes
Tsunamis
Detection—very difficult to detect in open ocean— because of the small wave height but long wave length. Tsunami waves have no back- only wind driven waves do.
Run-Up - if the first part of the wave to reach the coastline is the wave trough. -There may be a lowering of sea level below normal this is called DRAWDOWN—if this is recognised then can save lives eg
As the wave approaches land, the waves energy is crowded into a smaller volume of water, therefore waves that were 1m in height in the open ocean may reach 20m
Landfall- death and destruction will depend on what? Land uses, population density, warning given, geography and relief of coastal areas
Hydrostatic- objects like boats and vehicles are lifted and carried inland. Same could occur with a backwash/rundown
Hydrodynamic- tearing of buildings apart, washing away soil, undermining foundations
Shock effects- battering by debris carried in the wave—human deaths result from drowning, hit by moving debris, lifted and battered.
Hazard hits quickly and unexpectedly but prolonged—no time to think through properly in responses- so danger wasn’t recognised
Looks like a normal wave at first BUT on reaching land , breaks and floods due to one wave with no back
Occur from earthquakes of magnitude > 6.5
Occur when the focus has a depth of < 50km
90% occur in Pacific
Over 25% occur in the Japan-Taiwan island arc (the most active source area)
Between 1900 – 1980, 370 tsunami were observed in Pacific
The greater the tsunami run up (wave height above sea level) the greater the devastation but the less frequent
If the first part of the wave to reach the coastline is a wave trough, there may be a lowering of sea level below normal levels, called a drawdown.
Human factors affecting vulnerability to tectonic hazards
Population density |
Awareness |
Cultural factors affecting public response |
Access to education |
Lines of communication |
Early-warning system |
Emergency service |
Building codes |
Insurance |
Physical factors affecting vulnerability to tectonic hazards
Rock type |
Magnitude |
Frequency |
Time since the last hazardous event |
Location of epicentre |
Time of day |
Duration of shaking |
Depth of earthquake |
Relief of the land |
2002 Eruption of Mount Nyiragongo, Democratic Republic of the Congo
General information:
Divergent plate boundary on the East African Rift (African Plate splitting into the Nubian Plate and the Somali Plate); on the border of DRC and Rwanda
Height of 3470m; 2000m above a rainforest, approximately 15km from city of Goma (population approximately 700,000)
Opening is 1.2km in diameter, 600m deep lava lake at 1000⁰C, largest in the world
2002 eruption:
17th January 2002, 09:30, lasted roughly 24 hours
147 deaths reported by the UN, 60-100 of whom died in the explosion of Goma Central Petrol Station
470 people suffered burns, fractures, gas intoxication
14,000 homes destroyed, 30,000 people displaced
Up to 350,000 fled from the lava flows (covering 13% of the city) to Rwanda
Impacts:
CO2 seeping from rock pores endangers settlements into the future
Lack of toilets mean ‘long-drop’ toilets are necessary, allowing the spread of cholera
Digging toilets and graves in the rock is nearly impossible
Responses:
US gave $50,000 to DRC and Rwanda; $5 million to NGOs and the UN for disaster aid; wool blankets, water, dust masks, plastic sheeting (for shelter) worth over $800,000
Caritas (disaster relief NGO) Goma distributed food and relief for 15,000 families
ECHO (European Civil Protection and Humanitarian Aid Operations) gave €5 million
Concern Worldwide, UN/UNICEF, governments of 24 nations all donated
Current monitoring strategies:
Lava characteristics can’t be directly tested as samples are too hot and the journey to the crater is treacherous
A volcanic observatory was established
‘Goat Test’ to test CO2 levels
Time lapses on camera traps to measure changing level of lava lake which can show pressure changes in the magma; hard to see past smoke, harsh conditions bad for camera
Practice eruption drills for the population
SO2 levels monitored by gas box; unreliable as they depend on weather and wind direction
Analysis of lava bombs give information about lava for future eruptions
Microphones pick up low-frequency infrasound and monitor lava lake pressure changes
Deggs model
Earthquake predictions and problems
satellite geodesy- measure how the earth is deformed
good at knowing where earthquakes occur but not why
a useful prediction= size and level of damage, where and when
US geological survey locates about 55 every day
occur because of nucleation on slip on faults typically 20-30km deep
Neither the USGS nor any other scientists have ever predicted a major earthquake
how much force has been building up on the rock is unknown
we don’t know what the critical point of release is
some EQs have pre-cursors e.g. foreshocks, radon gas release, changes in animal behaviour but these are not consistent and only recognised as a precursor afterwards!
Japan, 2011 ground moved by 50m! as a result of the release of elastic energy
Examples of modifying the event strategies
Land use zoning- prevention of buildings on low-lying coasts (tsunamis), avoiding areas close to volcanoes, avoiding areas where liquefaction is likely
Benefits- low cost, relocates people from areas of high-risk
issues- prevents economic development in some coastal areas, requires strict enforcement
Aseismic buildings- cross-bracing, using counterweights, deep foundations e.g. Hawaii timber houses> can be moved easily
Benefits- protects people and property, financially possible in the developed world, basic design can be replicated in the developing world.
issues- high costs for tall buildings, older buildings and homes for people on low incomes are too difficult to protect
Tsunami defences- building sea walls and breakwaters
benefits- reduces damage, provides a sense of security
issues- can be overtopped, very high cost, unsightly
Tsunami resistant designs
Mangrove swamp and coral reef protection
Build buildings at a higher level far from the shoreline and not at the top of a smooth shallow beach
If buildings are high, then water can flow under them
It helps if the building is not square on to the wave front. If diagonal, the wave hits the pointed corner first and is diverted around the sides. Pressure is much reduced. Buildings should not be close together in a way that makes a wider dam. If roads have buildings all along both sides, the water is funnelled along the roadway, accumulating debris as it goes, and with no reduction in height or destructive force. It is much better if gaps are left between buildings out through which the water can dissipate. If the soil is sandy, then the footings should be deep and bracing should go right down to the feet. Light soil will also be protected from erosion by tarmac or concrete surfacing, which should go right underneath the floor if it is raised.
Timber buildings are much liked in earthquake areas because they are light and thus reduce earthquake effects. But they are the worst possible choice in tsunami-prone areas; like the ships, they float, and there is nothing to hold them down. The wood becomes weapons which destroy buildings and lives.
Lava diversion- channels, water cooling e.g. 1973 Eldfell, 2024 Blue Lagoon, 1983 Etna
Benefits- diverts lava away from people and buildings, relatively low-cost
issues- only works for basaltic lava, not feasible for majority of explosive volcanoes
Cry wolf syndrome
occurs when predictions prove to be wrong so that people are less likely to believe the next prediction and warning and therefore fail to evacuate
e.g. l’Aquila, Italy 2009- Scientists charged with manslaughter for failure to warn people
Earthquake kits
Governance
the sum of the many ways individuals and institutions, public and private, manage their common affairs. This involves negotiating responses to problems that affect more than one state or region.
Reason’s Swiss Cheese Model
really good in a conclusion
Modify the vulnerability and resilience strategies
HI tech
e.g. Sakurajima Japan
Community
e.g. 1997 Montserrat
1st September every year in Japan- Disaster Preparedness Day
Shakes out 3rd Thursday of October in California
Modify the loss strategies
Assess the importance of prediction and forecasting in reducing the vulnerability of communities to earthquake hazards. (12)
Factors affecting vulnerability Nepal 2015
Factors affecting vulnerability Japan 2011
Factors affecting vulnerability Haiti 2010
Assess the importance of governance in the successful management of tectonic mega-disasters
Intro: Define tectonic mega-disasters, What is management in relation to tectonics? consider both sides of why governance is good and why it can be bad, BLUF- Other factors such as physical can play a role but governance is most important alongside development- could put that in conclusion. Management involves preparation, prediction, prevention, damage control, and repair after the disaster. A good government is involved in each of these.
Preparation
How a government helps- Japan- modify Vulnerability
How a government is not important-
Prediction
How they help-
How they are not important- We cannot predict Earthquakes, and some governments may have less advanced prediction technology
After the disaster
How they help- modify the loss through secondary effects e.g. cholera in Haiti
How they aren’t important-
Corruption versus Non Corruption
For example in Haiti there is a corrupt government that lacks funding, especially after the 2011 earthquake that caused damage to 120% of their GDP they would rely on other countries for repairs
Repair and good relations with neighbouring countries can gain aid in an immediate response
With the 2008 Sichuan earthquake, the Chinese government helped with the development of education of hazards and immediate response to manage the disaster
On the other hand, managing tectonic disasters
Conclusion
Note 
Flashcard (137)